My attempts to make a rapid prototyping machine that I will use to make parts for a machine that will be able to make parts for a copy of itself.

Sunday, 3 January 2010

Extruder broke already

Well my best attempt at making a reliable extruder again resulted in one that only lasted a few weeks! The brass worm pulley that was pushed onto a splined shaft worked loose while extruding PMMA.

PMMA is quite hard work to extrude, but probably no worse than HDPE. On reflection splines into brass are not going to hold the massive force that occurs at 2mm radius. A better idea would be to have a boss on the side of the pulley and use a set screw onto a flat on the shaft. I would also add smaller diameter bosses at each side to meet the centre rim of the bearings. That would automatically position the pulley dead centre.

But to do that I would have to make a new pulley cutting jig and redesign the motor bracket to be a bit wider. I would need a working extruder to make the new bracket of course, so I decided to bodge the existing design.

I drilled out the centre of the pulley to 6mm and then reamed it to 6.4mm. I then turned a steel hub from a piece of hex pillar. I made it about a tenth of a millimetre oversized, added a chamfer to the hole in the pulley and forced it in with a vice, creating a very tight fit.

I didn't trust that to hold on its own so I left a hex flange on the other side and soldered it to the brass: -

Certainly not my best soldering, but bodging is bodging. The hub is twice as wide as the wheel and steel is harder than brass, so it should have a much better grip on the splines. I don't know if it will last or not. The constant back and forward motion of the anti-ooze fix means that if anything is weak it gets worked loose.

With the repaired extruder I made a third lamp shade clip leaving 1mm of the acrylic rod left above the pulley, how lucky is that?

Then I pushed my luck too far. When I bought the 3mm PMMA rod I also got a 2mm rod to compare results. Stiffness of a rod is a fourth power on diameter I think, so 2mm filament is five times more flexible than 3mm.

This would certainly be feasible to use in coils as it has a similar minimum bend radius to 3mm PLA, we just need to find somebody to supply it in that form at a reasonable price. 2mm rods are even more expensive than 3mm rods, £1.24 on eBay as opposed to £1.49, but are only 44% of the volume!

I decided to give it a try in my newly repaired extruder by printing a whistle. I had to scale it down because with 0.4mm filament it would use more than 1m of 2mm filament, so I printed the same g-code using 0.3mm filament and scaled the dimensions accordingly.

It managed to print a couple of layers and then the extruder jammed. I think the problem is that with a 3.6mm bore and 2mm filament there is too much of a gap, so molten plastic can flow upwards and freeze in the cold part of the tube above the taper. I think it would work fine with an extruder designed for 2mm filament. The drive mechanism just about works because although it does not have as much grip, it only needs 44% of the force that 3mm filament needs. The barrel and heater block would need a smaller bore though and could be made smaller. Similarly the smaller motor I used before would have plenty of torque, in fact a high torque NEMA14 should work.

So there are a lot of advantages to using 2mm feedstock like commercial machines do, BUT stiffness falls as a forth power, but force required only falls as a square law, so I expect soft plastics like HPDE, PP and PCL may buckle when being fed. Certainly the gap between the pinch wheel and the barrel entrance would need to be very small.

I fixed the jam by putting a drill down the hot barrel and hitting it with a hammer. That fixed it and I hand fed some ABS before reassembling the extruder. After assembly it would not work at all. The thermistor had shorted out to the metal work!

Nothing much to see from the outside, just a weird furry slimy deposit on the back of the AL tube and a green stain on the thermistor lead that was shorted.

I cannot get to the thermistor or heater without removing the PTFE cover, but that can't be removed without unscrewing the barrel, another slight design flaw. If I had tapped the stainless steel pipe all the way up I could just unscrew it from the AL tube that surrounds it, but it is really hard work tapping stainless steel.

I unscrewed the barrel while the extruder was hot to reveal this mess: -

The plastic that leaked when I first built the extruder has been stewing for weeks and has boiled down to something resembling bitumen. I expect the more volatile products condensed on the cold AL tube above it forming the Vaseline like deposit.

I couldn't tell why the thermistor was shorted because it came away with the PTFE cover. The Cerastil that I glued it in with seems to have decomposed in the chemical soup around it. My last few attempts at sticking thermistors with Cerastil have not been very successful. I am not sure if I mixed it to the wrong consistency, or if it is now too old to cure properly. It doesn't look any different, but instead of rock hard cement I seem to get something crumbly.

I cleaned it all up and stuck the thermistor back in with RTV silicone. I am sure it is not as conductive as Cerastil, but over such a short distance (between the thermistor and the wall of the hole it is in) I am hoping it will not have much effect.

I made the hole for it a bit deeper and opened out the top so it was big enough to accommodate the PTFE sleeving as well. That should keep it from touching the metal. It is surprisingly difficult to glue something into a small hole with a viscous glue. It is hard to get the glue to go down the hole without leaving an air pocket. A better idea might be to drill out a small screw, all the way through, fill it with glue from both sides. Then when it has set simply screw it into a tapped hole in the heater block.

I am waiting 24 hours for the silicone to cure now, so back to work tomorrow and less blog posts.

5 comments:

What kind of Cerastil are you using? I glued my thermistor with Cerastil C-3 fine grain grade. They say on room temperature it will cure in 7 days, but you can just heat it up at ~200°C and will cure very fast.

Have you ever tried to use a halogen light bulb for the heater ?There were small ones with 5, 10 and 20 Watts at 12 Volts with a G4 socket and they can fitted in an hole diameter of ca. 8.5 - 9 mm.The thermal flux is mainly done by radiation then by conduction in the IR-band, so no glue/cement/thermal compound should be required.

Thermistor fixing: There were black (thermo)paintings for oven pipes heatproved to 400 deg C. (dont know if they are electrically conductive).

Erhard, No I haven't tried a bulb but they are bigger and making connections is a bit more difficult. I don't need to use any compound with the resistors that I use. Just wrap then in AL foil so that they are a tight fit.

A bulb might give a faster response time though.

I have tried to use high temperature paint to stick nichrome heaters but it was not very good used as an adhesive.